On Sun, Oct 11, 2020 at 1:29 AM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote: > > Userfaultfd in unprivileged contexts could be potentially very > useful. We'd like to harden userfaultfd to make such unprivileged use > less risky. This patch series allows SELinux to manage userfaultfd > file descriptors and in the future, other kinds of > anonymous-inode-based file descriptor. SELinux policy authors can > apply policy types to anonymous inodes by providing name-based > transition rules keyed off the anonymous inode internal name ( > "[userfaultfd]" in the case of userfaultfd(2) file descriptors) and > applying policy to the new SIDs thus produced. > > With SELinux managed userfaultfd, an admin can control creation and > movement of the file descriptors. In particular, handling of > a userfaultfd descriptor by a different process is essentially a > ptrace access into the process, without any of the corresponding > security_ptrace_access_check() checks. For privacy, the admin may > want to deny such accesses, which is possible with SELinux support. > > Inside the kernel, a new anon_inode interface, anon_inode_getfd_secure, > allows callers to opt into this SELinux management. In this new "secure" > mode, anon_inodes create new ephemeral inodes for anonymous file objects > instead of reusing the normal anon_inodes singleton dummy inode. A new > LSM hook gives security modules an opportunity to configure and veto > these ephemeral inodes. > > This patch series is one of two fork of [1] and is an > alternative to [2]. > > The primary difference between the two patch series is that this > partch series creates a unique inode for each "secure" anonymous > inode, while the other patch series ([2]) continues using the > singleton dummy anonymous inode and adds a way to attach SELinux > security information directly to file objects. > > I prefer the approach in this patch series because 1) it's a smaller > patch than [2], and 2) it produces a more regular security > architecture: in this patch series, secure anonymous inodes aren't > S_PRIVATE and they maintain the SELinux property that the label for a > file is in its inode. We do need an additional inode per anonymous > file, but per-struct-file inode creation doesn't seem to be a problem > for pipes and sockets. > > The previous version of this feature ([1]) created a new SELinux > security class for userfaultfd file descriptors. This version adopts > the generic transition-based approach of [2]. > > This patch series also differs from [2] in that it doesn't affect all > anonymous inodes right away --- instead requiring anon_inodes callers > to opt in --- but this difference isn't one of basic approach. The > important question to resolve is whether we should be creating new > inodes or enhancing per-file data. > > Changes from the first version of the patch: > > - Removed some error checks > - Defined a new anon_inode SELinux class to resolve the > ambiguity in [3] > - Inherit sclass as well as descriptor from context inode > > Changes from the second version of the patch: > > - Fixed example policy in the commit message to reflect the use of > the new anon_inode class. > > Changes from the third version of the patch: > > - Dropped the fops parameter to the LSM hook > - Documented hook parameters > - Fixed incorrect class used for SELinux transition > - Removed stray UFFD changed early in the series > - Removed a redundant ERR_PTR(PTR_ERR()) > > Changes from the fourth version of the patch: > > - Removed an unused parameter from an internal function > - Fixed function documentation > > Changes from the fifth version of the patch: > > - Fixed function documentation in fs/anon_inodes.c and > include/linux/lsm_hooks.h > - Used anon_inode_getfd_secure() in userfaultfd() syscall and removed > owner from userfaultfd_ctx. > > Changes from the sixth version of the patch: > > - Removed definition of anon_inode_getfile_secure() as there are no > callers. > - Simplified function description of anon_inode_getfd_secure(). > - Elaborated more on the purpose of 'context_inode' in commit message. > > Changes from the seventh version of the patch: > > - Fixed error handling in _anon_inode_getfile(). > - Fixed minor comment and indentation related issues. > > Changes from the eighth version of the patch: > > - Replaced selinux_state.initialized with selinux_state.initialized > > Changes from the ninth version of the patch: > > - Fixed function names in fs/anon_inodes.c > - Fixed comment of anon_inode_getfd_secure() > - Fixed name of the patch wherein userfaultfd code uses > anon_inode_getfd_secure() > > [1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@xxxxxxxxxx/ > [2] https://lore.kernel.org/linux-fsdevel/20200213194157.5877-1-sds@xxxxxxxxxxxxx/ > [3] https://lore.kernel.org/lkml/23f725ca-5b5a-5938-fcc8-5bbbfc9ba9bc@xxxxxxxxxxxxx/ > > Daniel Colascione (3): > Add a new LSM-supporting anonymous inode interface > Teach SELinux about anonymous inodes > Use secure anon inodes for userfaultfd > > fs/anon_inodes.c | 148 ++++++++++++++++++++-------- > fs/userfaultfd.c | 19 ++-- > include/linux/anon_inodes.h | 8 ++ > include/linux/lsm_hook_defs.h | 2 + > include/linux/lsm_hooks.h | 9 ++ > include/linux/security.h | 10 ++ > security/security.c | 8 ++ > security/selinux/hooks.c | 53 ++++++++++ > security/selinux/include/classmap.h | 2 + > 9 files changed, 210 insertions(+), 49 deletions(-) > > -- > 2.28.0.1011.ga647a8990f-goog > Any suggestions on how to get VFS folks' (already CC'ed) attention on this patch series? In the meantime, I humbly request the SELinux/LSM/UFFD reviewers/maintainers to provide their reviews.
